A measuring apparatus of this kind with the electrochemical sensor corresponding thereto is disclosed in published German patent application 2,911,548. In this measuring apparatus, the measurement-sensitive surface of the sensor is covered by an exchangeable cap in which several pass-through openings are provided through which the gas to be detected passes to the diffusion membrane thereby making the gas accessible for measurement. In this way, the cap acts as a diffusion barrier which must be exchanged depending upon the concentration of the gas to be detected to thereby prevent the sensor from becoming overloaded with increased concentrations. For this purpose, a plurality of diffusion barriers with different numbers of diffusion openings and/or different cross sections of the diffusion openings are provided. The measuring range must be changed when the quantity of the gas to be detected is either below or above the detection limit of the measuring range then under consideration. If the measuring range must be changed, then the corresponding diffusion barrier must be exchanged. The lower limit of the measuring range corresponds to the detection limit; whereas, the upper limit of the measuring range for the particular diffusion barrier then seated in the apparatus is determined by the quantity of the gas to be detected which is transported in and which can still pass through the diffusion openings. If the quantity of gas transported in exceeds the diffusion capacity of the diffusion barrier, then the measuring apparatus will indicate a measured value which is too low. Furthermore, the characteristic of concentration versus measurement signal is then no longer linear.
In this known apparatus, it is a disadvantage that the adaptation to the different measuring ranges must be carried out manually so that intervention at the sensor takes place each time. Furthermore, the need for an exchange can only then be recognized when the measuring range has already been exceeded. These two requirements interrupt the measurement readiness of the measuring apparatus and make an effective display too slow. The sensor cannot provide a clear signal when the particular measuring range is exceeded in the presence of rapid changes in concentration without the diffusion barrier being first exchanged.